Pakistani national Majid Khan pleaded guilty on Wednesday at a Guantanamo military tribunal in a landmark case that could speed the trials of September 11 suspects.

Khan, 32, a protege of September 11 mastermind Khalid Sheikh Mohammed, pleaded guilty to conspiracy, murder and attempted murder in violation of the laws of war, and to material support for terrorism and espionage.

Dressed in a dark suit and pink tie, Khan spoke in fluent English without the aid of an interpreter.

He denied he ever meeting or speaking with slain Al-Qaeda leader Osama bin Laden, but admitted to taking part in a 'conspiracy' in Pakistan, Thailand and Indonesia.

Khan, who has spent the last nine years behind bars and faced possible life in prison, will be sentenced to no more than 19 years as part of a plea agreement that requires him to cooperate with US authorities, Judge James Pohl said.

In exchange for the lighter sentence, Khan will testify against other 'high value' detainees, including Mohammed and four others alleged to have taken part in the 2001 attacks.

To insure he fulfills his side of the bargain, the sentence will not be handed down for four years, until February 29, 2016.

Following an interruption by censors in the delayed transmission of the session, Khan told the judge he understood the terms of the plea deal.

"This agreement doesn't guarantee me that I'll go free. Basically I'm making a leap of faith, that's all I can do," he said.

It is the first plea bargain among 14 Guantanamo detainees the US military classifies as "high value."

"It's part of a strategy of building more solid cases against the handful of defendants that the government plans to try before the commissions," said Jonathan Hafetz, a lawyer who has represented other Guantanamo detainees.

More than 10 years after the September 11 attacks, Mohammed and four co-defendants accused of plotting the strikes are still awaiting trial at the prison, part of a US naval base in Cuba.

At Wednesday's hearing, Pohl rejected a defense request that the terms of the plea agreement be kept secret for security reasons, saying 'the nature of plea and the nature of information are already in the public domain'.

Khan is accused of working under Khalid Sheikh Mohammed's direction to plan explosions of fuel tanks at US gasoline stations and an attempted assassination of former Pakistani president Pervez Musharraf.

He is also accused of delivering funds for a bomb attack at a Marriott hotel in Jakarta, Indonesia, that killed 11 people in August 2003.

Round-faced with a small goatee and glasses, Khan lived in the United States for seven years before his arrest in Pakistan in 2003.

"If Khan provides information on KSM and others, as has been suggested was part of the deal, it will no doubt speed up the prosecutions," said Karen Greenberg, a terrorism expert at Fordham Law School, referring to Mohammed, who had been scheduled to be arraigned earlier this month.

Such testimony, she said, "will break through the barriers presented by evidence obtained through torture, as this information will be presented in the present time and in a legal proceeding."

US President Barack Obama -- criticised for failing to live up to his promise to shut down Guantanamo's prison by 2010 -- could benefit from the speedy prosecutions as he seeks a new term in November elections.

Over the years, 779 inmates have been detained at Guantanamo, most without charge or trial. Most have been transferred to their home countries or third countries in recent years and released.

Today, 171 people are still languishing there in limbo, including 89 detainees who have been cleared for release but are still in custody, thanks to a law passed by the US Congress.

For the other detainees who remain, pleading guilty may be the only way to guarantee that they one day leave the facility.

"The irony is that if you're charged with a crime and make a plea deal, you know you'll be released someday and have some idea when. You have an end-point," says David Remes, who has represented several detainees.

"But if you're not accused of a crime, you don't know whether you'll ever be released, much less when. That may be the crueler fate. The system is upside-down."

Khan was imprisoned in a secret CIA jail for three years before being transferred to Guantanamo in 2006.

He could be asked to testify in the trial of Abd al-Rahim al-Nashiri, the alleged Saudi mastermind of the 2000 attack on the USS Cole in Yemen's port of Aden, which killed 17 sailors and wounded another 40.